This invention relates to a carbon black reactor comprising; a combustion zone for generating a hot combustion gas by burning a mixture of a hydrocarbon fuel and an oxygen containing gas, a reaction zone for producing a carbon black by contacting the hot combustion gas with a feedstock introduced in a plurality of divided flows to thermally decompose and/or incompletely combust the feedstock, and a quench zone for stopping a carbon black growth reaction by injection a coolant to said carbon black containing flow, and these three zones are arrayed laterally in order, wherein the cross-section shape of the feedstock introduction part in the reaction zone is not a circle nor a polygon. The carbon black reactor can produce the carbon black having same characteristics, especially the distribution of the aggregate size, as one produced by pilot-scale reactor, while its production scale is increased to the level of that of reactor actually operated.